This invention relates to a radio transmitter effective to produce a coded signal. More particularly, the invention relates to such a radio transmitter which is particularly suitable for utilization in remote control operations; for example, to effect the opening of a garage door mechanism. The coded signal can be used to identify individual transmitters and thus be useful in security applications.
In many remotely controlled actuator applications, such as automatic garage door openers and the like, it has long been desirable to produce a light-weight compacted radio frequency signal transmitter which may be carried in a motor vehicle and transmit a coded signal. Generally speaking, a tuned receiver associated with the actuator receives the coded signal and initates the operation of the actuator to either open or close a garage door or the like. It is desirable that such transmitters be able to transmit a coded signal so that a plurality of similar actuators can be utilized in a comparatively small geographic area, without the disadvantage of one coded radio transmitter undesirably effecting the operation of more than one actuator.
The transmitter, when used to control actuators, typically is required to transmit intermittenly a signal over comparatively short distances, for example 300 yards, thus it can be made relatively small.
In coding of the radiated signal, it has been known to provide that the frequency of the radiated radio frequency signal be made variable, within limits, such as between 250 and 300 megacycles which provides a first parameter for tuning the receiver which is to be associated with the actuator. As a second parameter, it has been proposed to amplitude modulate by an audio frequency signal, the radio frequency signal with a selected particular audio frequency, controllable and within the range of, for example, from about 12 to about 24 kilocycles. By selecting a particular radio frequency signal and a particular audio frequency signal, a comparatively significant number of such transmitters may be utilized in a comparatively small geographic area.
A radio frequency transmitting arrangement of the type mentioned above is disclosed in U.S. Pat. No. 3,270,284 granted on Aug. 30, 1956 to W. A. Schanbacher.
It has been recognized that the selection of a single modulating audio frequency and a particular radio frequency within a given band may not provide a sufficient number of possible permutations to allow a considerable number of transmitters in a limited geographic area. In order to provide for additional permutations, it has been proposed to utilize a dual modulation of a remote control transmitter. For example, it is known from the U.S. Pat. No. 3,316,488 granted on Apr. 25, 1967 to W. S. Reynolds to provide a remote control transmitter with a modulation arrangement which effects modulation of the radio frequency signal with two distinct, different audio frequency signals, one of the audio frequency signals being supplied for a given period to the exclusion of the other audio frequency signal while the other audio signal is supplied subsequently to the exclusion of the one audio signal. Thus, the transmitted coded signal has three distinct parameters which can be utilized to identify the particular transmitter involved.
It is known from the U.S. Pat. No. 3,914,711 granted Oct. 21, 1975 to C. R. Carlson et al. to construct a gated oscillator from a NAND circuit, which is associated with two inverters and other circuit elements.